It is known in the art relating to two cycle engines having cylinder exhaust ports that the blowdown of pressurized gas in a cylinder when the exhaust port opens produces a high velocity jet directed from the cylinder into the exhaust passage. Since only a small portion of the exhaust port is open during blowdown, the area available for exhaust flow is only a small fraction of the total exhaust port area. Thus, the energy in the high velocity jet is dissipated by a sudden expansion.
It is also known that various exhaust port valve devices have been proposed for varying the timing of the exhaust process.
The present invention was developed from my recognition that the dissipation of the exhaust blowdown energy represents a loss, some portion of which might be recoverable by appropriate changes in the exhaust port design. The potential value of incorporating means for varying exhaust blowdown timing was also considered.